The dual capacity compressor is a kind of reciprocating type compressor of which piston stroke and compression capacity are made to change with rotation directions of a motor and a crankshaft by means of an eccentric sleeve rotatably coupled with a crank pin of a crankshaft. Since the dual capacity compressor has a compression capacity that can be changed depending on a required load, the dual compressor is used widely in apparatuses which require compression of working fluid, particularly in home appliances operative in a refrigeration cycle, such as a refrigerator, for enhancing an operation efficiency.
A U.S. Pat. No. 4,236,874 discloses a general dual capacity compressor, referring to which a related art dual capacity compressor will be described, briefly.
FIG. 1 illustrates a section of a dual capacity compressor disclosed in the U.S. Pat. No. 4,236,874, and FIG. 2 illustrates operation of the dual capacity compressor, schematically.
Referring to FIG. 1, the dual capacity compressor is provided with a piston 7 in a cylinder 8, a crankshaft 1, a crank pin 3 having an axis 3a eccentric from an axis 1a of the crankshaft 1, an eccentric ring 4 coupled with the crank pin 3, and a connecting rod 6 connected between the eccentric ring 4 and the piston 7, as key components. The eccentric ring 4 and the connecting rod 6 are rotatable with respect to each other, as well as the axis 3a of the crank pin. There are release areas 9 in contact surfaces of the crank pin 3 and the eccentric ring 4 respectively, and a key 5 for coupling the crank pin 3 with the eccentric ring 4 in the release areas. The operation of the dual capacity compressor with respect to the compression capacity will be described. As shown in FIG. 2, in the dual capacity compressor, a stroke of the piston 7 is regulated by an eccentricity varied with a position of the eccentric ring 4, wherein, if a large capacity is required, the crank shaft 1 is rotated in a clockwise direction (regular direction) and, if a small capacity is required, the crank shaft 1 is rotated in a counter clockwise direction (reverse direction). In detail, FIG. 2A illustrates a moment the piston 7 is at a top dead center during a clockwise direction rotation, and FIG. 2B illustrates a moment the piston 7 is at a bottom dead center during a clockwise direction rotation, when the strokes Lmax are the greatest because the eccentricity are the greatest. FIG. 2C illustrates a moment the piston 7 is at a bottom dead center during a counter clockwise direction rotation, and FIG. 2D illustrates a moment the piston 7 is at a top dead center during a counter clockwise direction rotation, when the strokes Lmin are the smallest because the eccentricity are the smallest.
However, during the foregoing operation, there are centrifugal forces acting on the crank pin 3 and the eccentric ring 4 perpendicular to the shaft axis 1a and the pin axis 3a respectively, along planes containing both the shaft axis 1a and the pin axis 3a, and both the shaft axis 1a and a center 4a of gravity of the ring respectively, caused by their rotation round the axis 1a of the crank shaft. Therefore, different from FIGS. 2A and 2B, in cases of FIGS. 2C and 2D, as lines of actions are not on the same line, a local rotating moment is taken place at the eccentric ring 4 with respect to the pin 3 as a product of a vertical distance ‘d’ to the pin 3 and its own centrifugal force, acting in a direction the same with a direction (counter clockwise direction) of rotation of the crank shaft 1. Since the crank pin 3 and the eccentric ring 4 are members that can make relative motion to each other, the rotating moment causes a relative rotation of the eccentric ring 4 in a direction of rotation of the crank shaft 1, disengaging the key 5 both from the crank pin 3 and the eccentric ring 4, and leaving the eccentric ring 4 and the key 5 to move in the rotation direction as shown in dashed lines in FIG. 3. Moreover, as shown in FIG. 3, for an example, during clockwise direction operation, a pressure ‘P’ (a pressure of re-expansion of the working fluid) in the cylinder after compression pushes the eccentric ring 4 to a direction of rotation of the crank shaft 1, to cause the eccentric ring 4 to make a relative rotation with respect to the crank pin 3 in a rotation direction of the crank shaft. At the end, such a relative rotation makes operation of the compressor unstable, to fail to obtain a desired compression performance.
In fact, the relative rotation is occurred because the key 5 fails to latch both the crank pin 3 and the eccentric ring, perfectly. The key 5 rolls within the release area whenever the direction of rotation of the crank shaft is changed, to result in serious wear at respective contact surfaces, that shortens a lifetime of the compressor.
In the meantime, other than the U.S. Pat. No. 4,236,874, there are many patent publications that disclose technologies of the dual capacity compressors, which will be described, briefly.
Similarly, U.S. Pat. No. 4,479,419 discloses a dual capacity compressor provided with a crank pin, eccentric cam and a key. The key is fixed to the eccentric cam, and moves along a track in a crank pin when a direction of rotation of the compressor is changed. However, since the key can not latch both the crank pin and the eccentric cam, perfectly, the U.S. Pat. No. 4,479,419 also has unstable operation caused by the relative rotation.
U.S. Pat. No. 5,951,261 discloses a compressor having an eccentric part with a diameter of bore, formed across the eccentric part, and an eccentric cam with another bore with a diameter the same with the eccentric part, formed at one side thereof. A pin is provided to the bore in the eccentric part, and a compression spring is provided to the bore in the eccentric sleeve. Accordingly, when the bores are aligned during rotation, the pin moves into the bore in the cam by a centrifugal force, that couple the eccentric part and the eccentric cam, together. However, since the U.S. Pat. No. 5,951,261 is provided with only one bore in the eccentric cam, the U.S. Pat. No. 5,951,261 can couple the eccentric part and the eccentric cam together only when the compressor rotates in a particular direction. Moreover, an operation reliability can not be secured, since an exact movement of the pin from the eccentric part to the cam through respective bores is difficult.